In the last stage of a low pressure turbine or the second or third last stage thereof, generally the pressure is very low, so the steam as a working fluid is in the state of wet steam which contains liquefied microscopic water droplets (molecules of the vapor). The molecules of the vapor condensed and attached to a blade surface gather and form a liquid film on the blade surface. Then, the liquid film is torn by the working fluid main flow steam into coarse droplets much larger than the original molecules of the vapor and the droplets are splashed downstream. After that, the coarse droplets flow down while they are somewhat pulverized by the main flow steam but remain relatively large. Due to their inertial force, the coarse droplets cannot turn quickly along a flow path like gaseous steam and they quickly collide against the moving blade downstream, which may cause erosion of the blade surface or interfere with the rotation of the turbine blade, resulting in loss.
In the past, in order to prevent such erosion, the tip of the leading edge of the moving blade has been covered by an erosion shield made of a hard high-strength material such as stellite. However, the recent steam turbines tend to have longer blades in which the tip of a moving blade is very thin and from the viewpoint of workability, it cannot be always covered by an erosion shield. Furthermore, generally protection of the blade surface is not enough to prevent erosion, so it is combined with another countermeasure against erosion.
Generally, removal of liquid droplets is most effective in reducing the influence of erosion. For example, as described in Patent Literature 1 (JP-A No. H5 (1993)-106405) and Patent Literature 2 (JP-A No. H11 (1999)-336503), a stationary blade structure has been proposed in which slots for sucking the water film (drain) on a blade surface are formed on the surface of the stationary blade with a hollow portion (cavity) inside which the pressure is low. Both Patent Literature 1 and Patent Literature 2 adopt a structure that a plurality of slot lines, in each of which a plurality of slots are radially arranged in line at regular intervals, are arranged in the axial direction.